1. Field of the Invention.
The present invention relates to the field of blind fasteners, and more particularly to blind fasteners of high strength and high fatigue life as are used in aircraft manufacture and the like.
2. Prior Art.
The present invention comprises an improvement to the Blind Rivet Assembly with Locking collar and Rivet Stem disclosed in U.S. Pat. No. 4,012,984. That patent discloses a blind rivet assembly having a blind rivet stem extending through a hollow rivet so that it may be pulled, thereby to expand the tail of the hollow rivet. A collar on the stem has a head fitting in a locking groove within the grip length of the stem so that when the tail of the collar abuts a pressure element bearing against the head of the hollow rivet, it is bulged outwardly into a recess in the rivet head, thereby to interlock the rivet stem in the hollow rivet. A weakened portion, such as a break groove, is provided on the stem adjacent the bulged end of the collar whereby upon further pulling, the rivet stem breaks at the weakened portion, leaving the remaining part of the blind rivet assembly in the workpiece.
Such a blind rivet assembly has found substantial uses in aircraft construction and other applications. However, the design and operation of the locking collar of such blind rivets have certain characteristics which may result in loose stems upon installation or after some period of use, and which may result in the installed fastener exhibiting less than its full strength and fatigue life potential. In particular, the locking collar must fit within the locking groove and rivet sleeve for all combinations of tolerances of parts, with the worst combination being the largest allowable locking groove and the smallest allowable locking collar. Consequently, the locking collar will fit slightly loosely within the locking groove for all other combinations of dimensions, even when the parts are right on the nominal dimensions. Thus on installation of the blind rivet, the locking collar is forced toward the tail of the rivet sleeve, to the extent it has clearance with respect to the locking groove, so that the stem of the installed rivet may move slightly toward the rivet tail under conditions of high stress or vibration, thereby reducing the strength and fatigue resistance of the rivet. Further, because the skirt of the locking collar is effectively formed on installation by the buckling of the upper skirt portion of the locking collar, the folding or buckling action of the skirt necessitates a rather deep recess in the sleeve head. This results in an overlap between this recess and the locking groove of the mandrel. The result is that the whole lock ring may get squeezed out of its pocket when a downward load is applied to the stem, so that after a very short travel, the stem is completely loose.
The invention disclosed and described herein consists of embodiments thereof of Messrs John August Louw and William Howard Owens, disclosed and claimed in the patent application and already assigned to Allfast Fastening Systems, Inc., and an embodiment of Ralph Luhm.